In recent years, a glass optical element has been employed over an extensive range as a digital camera lens, optical pickup lens for DVD, camera lens for mobile phone and coupling lens for optical communications. Such a glass optical element is often manufactured by a press molding method wherein a softened glass material is compression-molded by a molding die.
The level of performances required of a glass optical element is getting increasingly higher to catch up with the trend for more compact size and higher accuracy of various types of optical equipment. At the same time, there has been an increasing demand for a further reduction in manufacturing costs. Studies are being made on a method where the optical surface and the circumferential side surface of the glass optical element are simultaneously formed by molding.
As one of such methods, a method is known wherein a molding glass material of a predetermined mass and shape is prepared, this molding glass material and a molding die having a side transfer surface for forming a side of the outer periphery of a glass optical element are heated to the temperature that allows deformation of the glass, and then a glass optical element is produced by compression molding (U.S. Pat. No. 4,481,023).
Two molding methods disclosed in U.S. Pat. No. 4,481,023 will be described with reference to FIG. 7 and FIG. 8. FIG. 7 and FIG. 8 are diagrams showing the cross section of a molding die in the step of compression molding. FIG. 7 shows a molding die used in the first method, while FIG. 8 represents the molding die employed in the second method. In the first method, a molding die made of an upper mold 1, lower mold 2, and side transfer member 3 as shown in FIG. 7 is employed, and the upper mold 1 moves downward until the glass fills up a cavity (a space having the shape of a glass optical element inside the molding die), whereby pressure is applied to the glass (hereinafter referred to as “conventional method 1”). The thickness of a glass optical element 4 is determined by the volume of glass. In the second method, a molding die made of an upper mold 5, lower mold 6 and side transfer member 7 as shown in FIG. 8 is utilized, and the upper mold 5 stops at the position in contact with a side transfer member 7. Since the cavity is not completely filled with glass, some portions of the free glass surface 9 not being restricted by the mold are contained in the glass optical element 8 in this molding method (hereinafter referred to as “conventional method 2”). The thickness of the glass optical element 8 is determined by the thickness of the side transfer member 7.
In the meantime, differently from the method of using the molding glass material of a predetermined mass and shape, a method of compression molding is proposed. According to this proposal, a molding die made up of an upper mold, lower mold and side transfer member is heated, and the molten glass is dropped onto the lower mold. While the temperature is in the range wherein glass can be deformed by pressure, compression molding is provided using a molding die (Unexamined Japanese Patent Application Publications No. 2003-292327 and US2004/0231362).
However, in the aforementioned conventional method 1, the upper mold moves down until the cavity is filled with glass. This has caused such problems as quality failures wherein burrs and nicks are produced on both ends on the side of the outer periphery (the boundary between the surface transferred by the upper mold and the side surface of the outer periphery, and the boundary between the surface transferred by the lower mold and the side surface of the outer periphery).
In the aforementioned conventional method 2, some portions of the free glass surface not restricted by the die are contained in the glass optical element, such a problem as a burr or nick does not occur. However, since the movement of the upper mold 5 is restricted by the side transfer member 7, this method involves a serious defect wherein pressure applied to the glass optical element cannot be maintained in the step of cooling and shrinkage. Thus, this method has been unable to produce an optical element having a high precision optical surface.
For the optical element having a small diameter or the optical element wherein the optical surfaces on the front and rear have a similar shape, there has been a demand for such an arrangement that both ends on the side surface of the outer periphery are shaped differently to distinguish between the front and rear of the optical element. However, in the aforementioned conventional methods 1 and 2, it has been difficult to make a difference in the shape of the aforementioned portion, and the shape of this portion cannot be used to distinguish between the front and rear of the optical element.
Further, the Unexamined Japanese Patent Application Publication No. 2003-292327 and 2004-339039 also fail to disclose a glass optical element manufacturing method that provides a high precision optical surface and prevents a quality failure such as a burr or nick from occurring on both ends on the side surface of the outer periphery.